This invention relates generally to micro-electo-mechanical components and, more particularly, to a micro-electro-mechanical (MEMS) varactor and a method of making and using thereof.
A variable capacitor, also known as a varactor, is widely used in many circuit applications. For example, varactors have been used for many years in the radio industry. Previously, these varactors were interdigitated metal leaves with variable overlap area and the dielectric was air.
With the advent of semiconductor devices, p-n junctions were used to form varactors. Since the space charge region of semiconductor p-n junctions varies approximately as the square root of the applied bias, variable capacitance in these varactors is obtained by simply changing the applied bias. However, this leads to the constraint of defining the varactor DC bias dynamically.
Varactors using MEMs technology have been constructed as disclosed at www.eecs.umich.edu/RADLAB/bio/rebeiz/Current Research.html, which is herein incorporated by reference. Unfortunately, these varactors have a capacitance ratio of only on the order of 1.5:1 and 2.5:1. Additionally, these varactors have a fairly large mass which limits their response time.
A varactor in accordance with one embodiment of the present invention includes a first capacitor plate, a second capacitor plate, at least one fixed charge holder, and a control electrode. The second capacitor plate is spaced from and movable towards and away from the first capacitor plate. At least one fixed charge holder with an imbedded charge is on at least a portion of the second capacitor plate. The control electrode is spaced from the second capacitor plate.
A method of using a varactor in accordance with another embodiment of the present invention includes applying a first potential with the first polarity to a control electrode and moving a first capacitor plate towards or away from a second capacitor plate based on the first polarity of the applied first potential.
A method of making a varactor in accordance with another embodiment of the present invention includes providing first and second capacitor plates with the second capacitor plate spaced from and movable towards and away from the first capacitor plate. At least one fixed charge holder is formed on at least a portion of the second capacitor plate. Charge is imbedded in the at least one fixed charge holder. A control electrode is provided that is spaced from the second capacitor plate.
A method for making a varactor in accordance with another embodiment of the present invention includes filling a first trench in a first insulating material with a first conductive material to form a first capacitor plate. A second insulating material is deposited on at least a portion of the first insulating material. A second trench is formed in a portion of the second insulating material which extends to the first insulating material and is located over at least a portion of the first capacitor plate. The second trench is filled with a first sacrificial material. A second conductive material is deposited over at least a portion of the first sacrificial material and the second insulating material to form a second capacitor plate. A charge holding film is deposited over at least a portion of the second capacitor plate. A third insulating material is deposited over at least a portion of the charge holding film. A third trench is formed in a portion of the third insulating material which extends to the charge holding film. The third trench is filled with a second sacrificial material. A third conductive material is deposited over at least a portion of the second sacrificial material and the third insulating material to form a control electrode. Charge is imbedded in the charge holding film. The first and second sacrificial materials are removed to form a chamber.
The present invention provides a varactor which uses a push-pull technique utilizing fixed static charge. This varactor has a wider dynamic range than prior varactors on the order of at least 4:1. Additionally, the present invention significantly reduces the overall mass of the varactor and increases the force used to modulate a movable capacitor plate compared to prior varactors. As a result, the present invention has a much faster response time than prior MEMS varactors. The present invention can be used in a variety of different applications, such as in wireless communications, global positioning system, and/or cell phones.